This section introduces aspects that may be helpful in facilitating a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
The American Dental Association (ADA) has suggested that a pressing issue with restorative dental materials is the service life of the materials. The service life of the materials has been shown to be affected, for example, by the patient, the procedure and materials related issues. Examples of materials related issues can include; strength, hardness, toughness, wear resistance, tolerance to water, dimensional stability, and color stability.
Conventional dental fillings can be made out of fast-settings pastes obtained by mixing solid-liquid components. For example, the dental fillings can be set by an acid-base reaction (e.g., cements) or by polymerization (e.g., resins). An example standard filling can be made of zinc phosphate and can include zinc oxide powder and about a 50% phosphoric acid solution containing aluminum (Al) and zinc (Zn). This example mixed material can set into a hard, rigid cement by forming an amorphous zinc phosphate binder on a tooth. The bonding can arise from penetration into mechanically produced irregularities on the surface of a prepared tooth (e.g., due to surface roughening). However, some interfacial leakages can occur due to the cement's porosity.
Another example of filling material is zinc polyacrylate (polycarboxylate) cement, which can be formed from zinc oxide and a polyacrylic acid solution. The zinc ion can form a crosslink polymer structure by crosslinking carboxyl groups to calcium (Ca) ions on the surface of a tooth. Glass ionomer cements can also based on polyacrylic acid or its copolymers with maleic or itaconic acids and can utilize calcium aluminosilicate glass powder rather than zinc oxide. Glass ionomer cements can be set by crosslinking polyacid with Ca and Al ions from the glass.
Another type of dental filling material can be resins, which are fluid monomers systems based on, for example, aromatic or urethane dimethacrylates. Ceramic fillers can be present to yield a composite material. The resin can be filled with inorganic materials, for example, quartz, borosilicate glass, lithium aluminum silicate, barium aluminum silicate, barium fluoride, ceramic materials, etc., to form composite materials. These inorganic fillers can range in size from about 0.04 to about 10 μm. The composite materials can be mixed with a hardening catalyst or photoinitializer, and then cured using UV-light. Because these composites have a relatively low viscosity (since they have a small amount of filler compared to the resin) the composites can be applied to the necessary tooth regions easily and can fill in smaller regions such as cracks in the tooth. However, since the composites have less filler than other dental materials, shrinkage can occur during curing, which can make these composites inappropriate for larger cavities. Two-component resins can polymerize on mixing through a two part organic peroxide-tertiary amine initiator-activator system in about three minutes. Materials containing diketone initiators can achieve polymerization in about thirty seconds by exposure to visible (blue) light energy.
In view of the above, there has been a growing interest in producing new dental fillers based on nanocomposites that will improve upon the products currently available for dental restoration. Such new dental fillers would be tailored to have superior mechanical performance, by controlling, for example, microstructure, size and distribution of nanofillers.